<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>固定</title>
  <style>
    .a{
      width: 100px;
      height: 100px;
      background-color: black;
      position: fixed;
      bottom: 0;
      right: 0;
    }
    .b{
      height: 1920px;
    }
  </style>
</head>
<body>
<div class="a"></div>
<div class="b"></div>
</body>
</html>
